The present invention relates to the field of three-phase power distribution networks. More specifically, the present invention relates to the field of identifying the phase of a cable in a three-phase power distribution network.
Electric power distribution networks are used by the electric utilities to deliver electricity from generating plants to customers. Although the actual distribution voltages will vary from utility to utility, in a typical network, three-phase power at high voltage (345,000 volts phase-to-phase) is delivered to multiple transmission substations at which transformers step this high voltage down to a lower three-phase voltage (69,000 volts phase-to-phase). This 69,000-volt three-phase power then feeds multiple distribution substations whose transformers further step down the voltage to the distribution voltage (12,470 volts phase-to-phase) and separate the power into three single-phase feeder cables. Typically, these feeder cables operate at 7,200 volts phase-to-ground. Each of these feeder cables branch into multiple circuits to power a plurality of local pole-mounted or pad-mounted transformers which step the voltage down to a final voltage of 120 and 240 volts for delivery to the commercial and residential customers.
Ideally, the utilities try to initially design the feeder circuits such that the current loads on each single-phase output of the three-phase transformer are equal. However, over time as new customers are added, one of the phases may become more heavily loaded than the others. To re-balance the loading, some of the branch circuits are moved from the more heavily loaded phase to the more lightly loaded phases.
To re-balance the loading, the phase of each cable in a distribution cabinet must be accurately known. Otherwise, a cable may be erroneously removed from a more lightly loaded phase and placed on the more heavily loaded phase. If this happens, the procedure will have to be repeated again, which will cause a second disruption in service to all customers on the branch being re-phased. In the worst case, adding a greater load to the more heavily loaded phase may cause the substation fuse to blow on that phase, resulting in a large power outage for all customers on the more heavily loaded phase.
Currently, to accurately identify the phase of a particular feeder branch, utility company personnel must physically trace a cable run back through various distribution cabinets until they reach a point in the distribution network at which the phase is definitely known. This can be a very time consuming process.
Various devices and methods have been described to assist in the phase identification of cables. Bouvrette, U.S. Pat. No. 4,626,622, proposes using modems and telephone lines to transmit a signal associated with the phase, at a point on the cable in which it is known, to another point on the cable in which it is unknown. Pomatto, U.S. Pat. No. 5,510,700, proposes essentially the same thing only using radio signals. However, both these techniques require calibration procedures and special training to be used effectively.
Accordingly, it is the object of the present invention to provide a new and improved method, of identifying the phase of a cable that is easy to use and does not require any calibration procedures or special training on the part of the user.
Briefly, to achieve the desired object of the present invention, Global Positioning System (GPS) receivers are used at both the known and unknown phase locations in the power distribution network to time-correlate phase information, thereby identifying the phase of the cable in question.
Specifically, using the 1 pps (one pulse per second) GPS signal, phase information for the known phase will be recorded at each GPS second and entered into a computer. A worker at the unknown phase location will simultaneously also record the cable phase information at a particular 1 pps GPS second on a phase identification meter that will convert the recorded time and phase to a short alpha or numeric sequence. The worker will then give that sequence to the dispatcher. The dispatcher will determine the phase by entering the sequence received from the worker into the computer. The dispatcher will then relay the cable phase designation back to the worker.
In this way, the worker determines the cable phase by taking a simple meter measurement in much the same way voltage and current measurements are taken. Therefore, no special procedures or training is required on the part of the worker.